There are many applications wherein a close clearance space is required between a moving member and a stationary member. For example, in designing turbines, a formidable problem is encountered when trying to minimize the clearance space between the turbine blade and the turbine housing. Although a close tolerance fit can be obtained by fabricating mating parts to within a close tolerance range, the expense required for such a fabrication process limits its use in commercial applications. In addition, when the mated assembly is exposed to a high temperature environment, the coefficient of expansion of the mating members may be different thus causing the clearance space to increase or decrease. The latter condition could result in a frictional contact between the members which in turn would cause a higher temperature to exist and thereby possibly damage one or both members. In the former condition, the increased clearance space in a turbine would permit gas to escape between the turbine blade tip and the housing thus leading to a decrease in efficiency since the escaping gas represents energy that has not been fully utilized.
Various coating techniques have been employed to coat the inside diameter of the turbine housing with an abradable coating which can be worn if frictional contact of the turbine blade should occur due to thermal expansion or growth of the rotating parts and/or non concentric distortion of the case. These coatings are intended to minimize rotor damage and gas leakage across a turbine stage if frictional contacts are experienced. This abradable coating technique for turbines not only increases the operating efficiency of the turbine but also provides a quick and inexpensive method for reservicing excessively worn seal members of the turbine.
Abradable seals presently available are predominately metallic in composition and hence cannot be employed in high temperature environments, but are suitable in situations where surface temperatures do not exceed about 1150.degree.C. Modern jet engines require seals capable of sustained operation at temperatures as high as 1650.degree.C. Ceramic materials will withstand such temperatures and can be formed in porous coatings so as to provide abradable seals. A pure ceramic seal lacks the necessary ductility, however, and cannot be satisfactorily bonded to the superalloy metal substrates commonly employed.
These requirements make impractical the direct bonding of a ceramic abradable seal to the substrates commonly used in turbine applications. Such substrates must be superalloys, which melt at about 1400.degree. to 1500.degree.C and the required refractory and noble metals are not sufficiently oxidation resistant to operate in the turbine. Another serious drawback to the present ceramic/metal bonding technique is the inherently thin bond that this process developes. The ceramic and metal usually have different coefficients of thermal expansion and upon thermal cycling the resultant strain must be absorbed at the bond area, which is thin and highly stressed. Thus this type of structure is clearly deficient for withstanding the thermal cycling necessary for turbine application. Nonetheless a seal structure must be provided which will behave as a thermal barrier to protect the metal backing from high temperatures, yet be sacraficial and "abrade" away when a rotating turbine blade contacts it.